Color photographic materials usually comprise a support and at least two silver halide emulsion layers having different color sensitivities (the term "color sensitivity" as used herein denotes the property of being sensitive to three wavelength regions of the spectrum of visible light, namely, any of red, green and blue light wavelengths) formed thereon.
In the field of color photographic materials, particularly color reversal photographic materials often utilized in practice by professional photographers, color photographic materials having high sensitivity are required in order to photograph certain scenes, for example, for photographing sporting events which requires fast shutter speeds, or photographing stage scenes in which the quantity of light required for exposure is often insufficient. Few color photographic materials are available which meet the above requirements. Under these circumstances, the sensitivity of the color photographic materials can be adjusted by treating the material in order to compensate for insufficient exposure. This adjustment of sensitivity by treatment is usually called "push development", and in the case of color reversal photographic materials, it is carried out by prolonging the time of the first development step (black-and-white development) beyond the time normally employed for standard processing.
However, conventional color reversal photographic materials do not always have sufficient push developability, and thus have one or more of the following defects:
(1) sensitization is not adequately effected unless the time of the first development step is prolonged well beyond the time required for standard processing;
(2) in a photographic material comprising a relatively high sensitive layer and a relatively low sensitive layer separated from each other, the push development may cause a change in gradation because these layers have different developability.
(3) an attempt to increase the degree of sensitization by prolonging the first development time results in a drastic decrease in the maximum density of the resulting colored images; and
(4) at the time of the push development, the color balance is degraded because of the differences in the developability among a red-sensitive layer, a green-sensitive layer and a blud-sensitive layer.
U.S. Pat. Nos. 2,996,382 and 3,178,282 describe a negative image-forming photographic element having an increased speed and contrast as a result of including both silver halide grains capable of forming a surface latent image when exposed, and silver halide grains containing an internal fog nucleus. The specificationsof these U.S. patents, however, fail to describe the push development, and also fail to disclose an ordinary color reversal photographic material. According to these types of photographic element, silver halide grains having a surface latent image release a reaction product according to the amount of exposure upon development after exposure. This causes the formation of cracks in the silver halide grains having an internal fog nucleus, and also enables development. The speed and contrast increase also during standard development of such photographic materials, but the sensitization cannot be controlled by the push development.
Japanese patent application (OPI) No. 214852/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a method of improving push developability by incorporating a silver halide emulsion having a fog nucleus in the interior of the silver halide grains into a photosensitive emulsion layer and/or a layer adjacent to it.
Investigations in connection with the development of the present invention, however, showed that when a color reversal photographic material included a silver halide emulsion containing a fog nucleus inside the silver halide grains in a photosensitive emulsion layer and/or a layer adjacent to it, sensitization of the lower density portion becomes greater than sensitization of the higher density portion at the time of push development. Also, the entire gradation during push development becomes undesirably hard.
It has been desired, therefore, to develop a color reversal photographic material in which at the time of push development, the sensitivity is desirably increased, the reduction of maximum density is small, and variations in gradation are also desirably small.